This invention relates to electron materials science and to microelectronics, including vacuum microelectronics, in particular to preparation and fabrication of devices based on field electron emission such as matrix field-emission cathodes, field-emission displays, microwave devices, various electron guns, etc.
Field emission is usually realized from sharp tips having radii of curvature in the range of micrometers and nanometers. There are several methods for their preparation.
One of them consists in growing of oriented, mutually parallel whiskers on a substrate and, then, sharp tips are prepared from the whiskers by various treatments.
To this aim, chemical vapor deposition of crystals, including chemical transport of substances, is most suitable. It is known a method for crystallization of materials in a narrow space [1]. However, this method is not effective for growing of oriented whisker arrays, because it does not ensure nucleation and growth of whiskers at particular points of substrates.
It is known a method for growing of oriented whisker arrays on a single-crystalline substrate oriented along the most-close-packed crystallographic face of a given material by vapor phase deposition, at heating, via solvent (liquidforming) particles orderly deposited on the substrate [2]. This method is based on the vapor-liquid-solid (VLS) growth mechanism proposed by the same author [3]. However, the method and apparatus patented and described for its realization suffer from the fact that the grown whiskers are often branching, kinking, etc.
In the scopes of the mentioned method for whisker growing, a procedure for creation of localized (e.g., micron-sized) solvent particles is important. For growing of, e.g., silicon whiskers, metals, such as gold, copper, nickel, etc, are used as solvents.
One of the technique for preparation of the localized particles consists in evaporation of the metals through a template mask. However, such a technique is unsuitable for localization of the particles on large-area substrates (for example, cm2 or more), because, in such a case, it is impossible to ensure a uniform clasp of the template to the substrate. On this reason, the metal particles formed are not clear-cut, have different sizes, etc.
More suitable for this is a photolithographic process. Such a process, as applied to whisker growth, is described in [4]. This method, however, gives a poorly-reproducible results because the solvent used (e.g., gold) contacts with the photoresist at a preliminary stage of the procedure resulting in a non-controlled, non-oriented growth of whiskers.
Another photolithographic method for localization of the solvent is described in [5]. There, the solvent (gold) is deposited inside an oxide mask formed on silicon substrate. However, the method suffers from the fact that the liquid alloy (silicon-gold in this case) is spreading, at high temperature, along the substrate-oxide interface, xe2x80x9cexplosesxe2x80x9d the oxide and, accordingly, an ordered whisker growth is destroyed.
It is known an apparatus for growing of oriented whisker arrays that consists of a tube reactor with a gas-mixture that flows through the reactor evolving the crystallized material, of an axis-symmetric substrate holder and a heating source [6]. The apparatus, however, has a figured. shape heater with cavities for placing of the substrates. Such a heater does not allow to reach an ordered, uniform, perfect growth of whiskers on large areas due to fluctuations of the gas streams, a non-uniform temperature gradients, etc.
Tasks of this invention are the following:
(1) A method for controlled growing of whiskers on a substrate that allows to prepare regular arrays of well-oriented whisker arrays on a large area. Thus method must also include techniques for deposition of arrays of localizes solvent particles on the substrate.
(2) Apparatus for realization of the method for controlled growing of the whisker arrays on a substrate that pprovides preparation of such arrays uniform on large areas.
The tasks indicated are realized in this method by growing of the oriented whisker arrays, preferentially for field electron emitters, on a single-crystalline substrate, oriented along the most-close-packed crystallographic plane of a given material, by vapor-phase deposition at heating, via solvent particles deposited onto the substrate orderly, wherein in parallel to the substrate is placed a solid-state source of the material for whisker growth with a plane surface faced to the substrate, the solid-state material source having the same composition as the growing whiskers, so that a vectorly-uniform temperature field, whose gradient is perpendicular to both the substrate and the source, is created between the substrate and the source. The solvent particles are deposited onto the substrate by either evaporation through a template mask or by means of a photolithographic technique.
The temperature of the material source can be higher than the temperature of the substrate. At such a case, in the space between the source and the substrate,- vacuum is created or an inert gas is introduced, and the material is transported from the source to the substrate by evaporation and condensation. At another case, the transport is ensured by introduction, in the space between the material source and the substrate, of a substance that reacts with them chemically.
The temperature of the material source can be lower than the temperature of the substrate. At such a case, in the space between the source and the substrate is introduced a substance that transports the material from the source to the substrate by means of a chemical reaction.
When the solvent particles are deposited by the photolithographic process, after creation of openings in the protection mask, hollows are formed in the substrate opposite to the openings. The diameter of the hollows exceeds the diameter of the openings, their depth is not less than 0.1 of the diameter of the openings, the solvent is deposited all over the substrate and is subsequently removed from all areas, except of the bottom of the openings.
The solvent deposited in the photolithographic process is removed from the surface of the protection mask either mechanically (by wiping) or chemically (by dissolution of the mask together with the solvent).
Silicon can be used as a material source and as a substrate, the latter should be a silicon wafer with the (111) crystallographic orientation. As a solvent, gold can be used, as an example. The whisker growing is performed at temperatures higher than 800xc2x0 C. As a substance that transports the material source, a mixture of hydrogen and silicone tetrachloride is used.
The task of this invention consists also in an apparatus for growing of oriented whisker arrays, preferentially for field electron emitters, that consists of a tube reactor with flows of gaseous mixtures evolving the crystallizing material, of a substrate, an axis-symmetrical substrate holder, and a heat source. In the tube reactor, opposite to the substrate a material source is placed that takes a heat from the heat source, whereas the substrate is heated from the material source.
In another version of the apparatus for growing of oriented whisker arrays, in the tube reactor, opposite to the substrate, a material source is placed, the substrate taking the heat from the heat source, whereas the material source is heated from the substrate.
The heat source can be done as a high-frequency inductor. The inductor can have a cylindrical or conical shape. The material source is placed on a holder made as a truncated circular cone with bases perpendicular to its axis. Larger basis of the cone has a cylindrical continuation, and the material source is placed on this basis, whereas smaller basis of the conical holder of the material source has a protrusion (projection). The substrate, opposite to the material source, is placed on a holder that is made as a heat racial: with a plane surface adjoining to the substrate and that H-as heat-radiation protrusions on the opposite side of the substrate holder along the edges of the neat radiator.
In another version of a similar apparatus, a substrate holder is made as a truncated circular cone with bases perpendicular to its axis. Larger basis of the cone has a cylindrical continuation, and the substrate is placed on this basis, whereas smaller basis of the conical holder has a protrusion. The material source, opposite to the substrate, is placed on a holder that is made as a heat radiator with a plane surface adjoining to the material source and that has heat-radiation protrusions on the opposite side of the holder of the material source along the edges of the heat radiator.
In the above apparatus, the cone is arranged concentrically inside the inductor, a part of the cone being out of the inductor, with the larger basis outside. The distance from the larger basis to the outside plane of the last coil of the inductor is 0.2 to 0.8 of the outside diameter of the inductor.
The angle of the cone is 15xc2x0 to 120xc2x0, whereas the protrusion of the conical holder has a cylindrical shape with the ratio of its diameter and of its height to the larger basis of the cone lies in the range 0.1 to 0.5.
In another version of the apparatus for growing of the oriented whisker arrays, the heat source is placed inside of an axis-symmetrical hollow holder of the material source, the holder having prismatic or truncated-pyramidal outside surface. The material source takes heat from the surface of the prism or pyramid, whereas the substrate takes heat from the material source, and a heat leveller (homogenizer) is adjoining by its plane surface to the substrate.
In one more version of the apparatus, the hear source is placed inside of an axis-symmetrical hollow holder of the substrates, the holder having prismatic or truncated-pyramidal outside surface. The substrates take heat from the surface of the prism or pyramid, whereas the material source Zakes heat from the substrates, and a heat leveller is adjoining by its plane surface to the material source.
In the both latter versions of the apparatus, wall thickness of the holders is 0.05 to 0.2 of the outside diameter of the prism or truncated pyramid.
In other versions of the apparatus for growing of the oriented whisker arrays, the heat source is made of lasers or lamps, or of their combinations, including combinations with high-frequency inductors.